The alkalinity of early soybean protein (soy) adhesives was high and was not suitable for hardwood plywood, with the high pH discoloring the decorative hardwood face. Soy adhesives were largely displaced in the plywood industry after World War II by urea-formaldehyde (UF) adhesives and phenol-formaldehyde adhesives due to cost and performance. Resurgence in soy adhesives has occurred in recent years due to concerns over the use of formaldehyde, which is listed as a known carcinogen.
An adhesive mix utilizing a polyamidoamine-epichlorohydrin (PAE) resin is currently used in the hardwood plywood industry. The PAE resins are known as thermally-setting adhesives. Thermally setting in this case means that to overcome the reactive energy barrier, a minimum temperature must be achieved. In the case of PAE resins, the azetidinium functional group is a ring structure. The ring must be opened for the PAE to react with the chemical components of the adhesive and adherent. The open and closed rings are in equilibrium. While a high pH will increase the percentage of open ring structures, too high of a pH makes the adhesive unstable and therefore the glue is not able to be processed in an industrial setting. PAE resins are stored at low pH to reduce the frequency of ring opening. A current practice relies on a stable adhesive with a pot life, i.e., remains pumpable, for hours or days, and then becomes set or reacted with the application of heat. The set gives the adhesive the required water resistance as outlined for Hardwood Plywood Association (HWPA) for Type II adhesives. However, the application of heat may increase energy requirements, increase fugitive emissions or off-gassing, and/or increase thermal stresses and/or defects such as warping, casehardening, or charring, which defects may particularly hinder the formation of decorative plywood.